In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric as water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The papermaking machine is, in essence, a water removal, system.
Temperature controlled rotating rolls are used throughout the papermaking process. For instance, a web or sheet of paper from the forming section and press section of papermaking machine is conveyed through a series of drying cylinders where additional water is removed and the web or sheet structure consolidated. In the finishing section, the paper typically contacts a series of water or oil-heated rolls where improved surface characteristics are imparted on the paper sheet. In some machines, the sheet is calendered through a series of rolls to allow for thickness control and to further smooth the surface. A heating device is typically used to selectively heat a calender roll surface to control the sheet caliper in the cross direction.
Maintaining the temperature profiles of the various rotating rolls, which including cylinders, at the desired temperature ranges is critical for process optimization and attendant product quality control. Most of the rolls have polished mild steel finishes, which can be covered with chrome, so the surfaces are very shiny and reflective of light. Infrared sensors, for example, have been employed to continuously measure the temperature profiles of these reflective surfaces. Unfortunately, implementing optical pyrometry techniques have proved to be a challenge in part because of variations in reflectively of the rotating surfaces and/or low emissivity of the surface materials. As a result, IR devices are very expensive and complicated. Using contacting temperature probes based on thermovoltaic or thermoresistive principles is not a viable option since the probes will erode the smooth rotating surface with the concomitant buildup of debris.